The data which computers can process are digital values, while the data used by humans are mostly analog values. Therefore, if the data used by humans are to be processed by computers, the data must be necessarily converted to digital values.
The ADC for converting analogue values to digital values has been developed in various typed depending on the application fields. The converting methods of the ADC can be classified into two types, i.e., an integration type and a comparison method, and the former is further classified into a voltage-time conversion type and voltage-frequency conversion type, while the comparison type is further classified into a feedback comparison type and a non-feedback comparison type. (1. Eugen R. Hentek, "A User's Handbook of D/A and A/D converter", DCA Reliability Laboratory Inc., 2. David P. Hoeschele, Jr., John Wiley & Sons, "Analogue to Digital/Digital to Analogue Conversion Techniques".)
The comparison type is very fast in its conversion speed compared with the integration type, and is applicable to fields such as video signal processings in which high conversion speeds are required. The full parallel-connected comparison type (flash type) ADC basically comprises a reference resistance row, voltage comparators, and an encoder. However, in forming such components into an integrated circuit, there are a number of drawbacks, such as an increase in chip area due to the existence of the resistance row, relatively large power consumption, a decrease in the operation speed, and the like.